This invention relates to wet process plating cells, either galvanic (for electroplating) or electroless (chemical plating), and is more particularly directed to a technique that permits the rapid insertion and removal of the workpiece to be plated into and from the cell. The invention also concerns a technique that facilitates employment of robotic means for transfer between stations of the articles to be plated.
Electroplating plays a significant role in the production of many rather sophisticated technology products, such as masters and stampers for use in producing digital compact discs or in the manufacturing of advanced semiconductor wafers. However, as these products have become more and more sophisticated, the tolerances of the plating process have become narrower and narrower. For example, in a modern CD, impurities or blemishes of 0.3 micron or larger can create unacceptable data losses. Current electroplating techniques can result in block error rates of 70, and with higher density recordings, the block error rate can be 90 or higher. Current plans to increase the circuit density of silicon wafers are being thwarted by the inability of plating techniques to control blemishes in the plating process.
A number of techniques for electro-depositing or coating on an article face been described in the patent literature, but it has been difficult to achieve the high plating purity and evenness of application that are required for super-high density optical media and semiconductor devices.
A recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent U.S. Pat. No. 5,597,460, granted Jan. 28, 1997. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or semiconductor wafer.
In the manufacture of compact discs, there is a step that involves the use of a so-called stamper. The stampers are negative discs that are pressed against the material for the final discs to create an impression that becomes the pattern of tracks in the product compact discs.
Stampers are nickel and are electroformed. The stampers are deposited on a substrate that has the data tracks formed on it, and has been provided with a conductive surface, e.g., by sputter coating. Then the substrate is placed into a plating tank. The nickel is introduced in solution into the process cell so that it can be electrochemically adhered onto the substrate surface, using standard electroplating principles. Present industry standards require the stamper to have an extremely high degree of flatness, and where higher density storage is to be achieved, the flatness tolerance for the nickel coating becomes narrower and narrower.
The flow regime for the plating solution within the tank or cell is crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the substrate in the plating cell.
Present day electroplating cells employ a simple technique to inject fluid into the process vessel or cell. Usually, a simple pipe or tube is used with an open end that supplies the solution into the tank or cell. The solution is forced from the open end of the pipe. This technique is not conducive to producing a flat coating, due to the fact that the liquid is not uniformly distributed across the surface of the workpiece. This technique can create high points and low points in the resulting plated layer, because of localized eddies and turbulences in the flow regime.
In the plating cell as described in said U.S. Pat. No. 5,597,460, a plating bath contains the electrolyte or plating solution, in which the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and forms a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. Adjacent the plating bath is an anode chamber in which anode material is disposed, with the material being contained within an anode basket. In a typical optical media or semiconductor electrolytic metallization process, the anode material is in the form of pellets, chunks or nuggets of metal, which are consumed during the plating process. A weir separates the plating bath from the anode chamber, and permits the plating solution to spill over its top edge from the plating bath into the anode chamber. The weir is in the form of a semipermeable barrier that permits metal ions to pass through from the anode chamber into the plating bath, but blocks passage of any particulate matter. A circulation system is coupled to the drain outlet to draw off the solution from the anode chamber, together with any entrained particles, and to feed the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is re-introduced into the plating cell. In this way a backwash of the plating solution is effected, so that the flow regime of the fluid itself washes any particulates out of the anode chamber in the direction away from the plated article. At the same time, the cleansed and purified solution bathes the plated surface of the substrate as a uniform, laminar flow of solution, thus avoiding high spots or voids during plating. As a result, very high tolerance is achieved, permitting production of compact disc or semiconductor device of extreme density without significant error rates.
The flow regime as described in said U.S. Pat. No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. In that patent the substrate can be positioned on either a fixed or a conventional rotary mount. A conventional cathodic motor rotates the substrate, e.g. at 45-50 RPM. The substrate can be oriented anywhere from vertical to about 45 degrees from vertical. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangement was intended to avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences.
A U-tube laminar flow sparger, shaped to fit on the lower wall of the plating bath or plating cell, can be positioned adjacent the base of the weir to flow the solution into the space defined between the substrate and the weir. The sparger's flow holes are directed in parallel to create a uniform, laminar flow of the electrolyte across the planar face of the substrate. The axes of the flow holes in the sparger define the flow direction of the plating solution, i.e., generally upwards and parallel to the face of the plated substrate.
Unfortunately, even with these improvements, the plating is not completely even over the substrate. There is a tendency for hydrogen bubbles to accumulate on the surface of the substrate where electrolytic plating is taking place, and these can interfere with the plating and cause errors in the metallized wafer. Also, with conventional plating there is a tendency for the plated surface to become bowed out, that is, for the plated metal layer to lose its flatness away from the center. Consequently, it was necessary to plate a large margin around the targeted substrate or stamper, so that center part will have the desired flatness. This necessitated using additional time and materials.
An improvement to this arrangement is described and illustrated in my earlier U.S. Pat. No. 5,683,564, which was granted on Nov. 4, 1997, which is incorporated herein by reference. According to that improvement, a rotary blade or wiper is positioned in the plating bath between the semipermeable membrane wall and the substrate, and has an edge disposed a predetermined distance from the planar face of the substrate. This distance can be about one-half inch, and is preferably about three-eighths inch. Preferably, the blade or wiper is pitched in the direction such that the rotating wiper tends to pull the electrolyte, plus any hydrogen bubbles, away from the substrate. The rotary wiper can be fluid powered, and as such can be coupled to the electrolyte return conduit so that the electrolyte itself serves as motive power. The fluid powered wiper can be formed with an annular turbine, mounted in a circular mount therefor that is disposed in the plating bath. A circular opening is in registry with the substrate face that is to be plated. The blade on the annular turbine extends radially inwards. The turbine can have vanes around its periphery, and the circular mount can have an annular recess around which the vanes travel. A conduit from the return conduit to the annular recess supplies fluid to propel the turbine and vane. As the same filtered and conditioned electrolyte that is fed through the sparger into the plating bath is also used to power the turbine, the leakage from this turbine does not in any way contaminate or dilute the electrolyte in the plating bath. The same materials that are used in the walls of the plating cell, e.g., a high quality polypropylene or PFA TEFLON.RTM. (polytetrafluoroethylene), are also used for the rotary blade, turbine, and mount. The annular turbine can be supported for rotation by rollers (formed of the same or a compatible plastic resin) mounted on the support for the annular turbine. This avoids the need for any bearings or metallic parts. In other possible implementations, a different motor mechanism could be employed to rotate the blade or wiper.
Electroless plating is favored in many applications, and especially in those where there is no electrically conductive layer that could serve as a cathode. Accordingly, electroless plating is now seen as an economical alternative to sputtering or vacuum deposition.
One advantageous approach to electroless plating is disclosed in my U.S. Pat. No. 5,865,894, which was granted on Feb. 2, 1999, which is incorporated herein by reference. In that arrangement, a megasonic transducer adjacent the floor of the plating cell applies megasonic energy at a frequency of about 0.2 to 5 MHz to the solution. The frequency can be above 1 MHz, and in some cases above 5 MHz. The megasonic waves distribute the solution evenly on the substrate, and also break up any bubbles or concentrations that may lead to defects in the plated surface.
Where the megasonic plating technique is used for electroplating silicon wafers, the flow regime is further improved by rotating the wafers. This can be achieved by placing the wafers in a carrier or boat and rotating the boat, e.g. at 45-50 RPM. This avoids regions of dead flow within the carrier, and results in uniformity of the metallization thickness and quality.
In order to employ the megasonic plating technique with a stationary substrate, the megasonic transducer and the rotary blade can be incorporated together in a plating cell, as described and illustrated in my U.S. patent application Ser. No. 08/954,239, which was filed on Oct. 20, 1997, is still pending and has been incorporated herein by reference.
To date, mounting the substrate and lowering the substrate into the plating cell have had to be done manually, and have not been automated or robotized. Automation and robotization of the insertion, removal, and transport of the workpiece from one process cell to another have been elusive and have not been realized. This has made it difficult to conduct the entire multiple step plating operation in a clean or super-clean environment.